bzero, explicit_bzero - zero a byte string
#include <strings.h>
void bzero(void *s, size_t n);
#include <string.h>
void explicit_bzero(void *s, size_t n);
The
bzero() function erases the data in the
n bytes of the memory
starting at the location pointed to by
s, by writing zeros (bytes
containing '\0') to that area.
The
explicit_bzero() function performs the same task as
bzero().
It differs from
bzero() in that it guarantees that compiler
optimizations will not remove the erase operation if the compiler deduces that
the operation is "unnecessary".
None.
explicit_bzero() first appeared in glibc 2.25.
For an explanation of the terms used in this section, see
attributes(7).
| Interface |
Attribute |
Value |
| bzero (), explicit_bzero () |
Thread safety |
MT-Safe |
The
bzero() function is deprecated (marked as LEGACY in POSIX.1-2001);
use
memset(3) in new programs. POSIX.1-2008 removes the specification
of
bzero(). The
bzero() function first appeared in 4.3BSD.
The
explicit_bzero() function is a nonstandard extension that is also
present on some of the BSDs. Some other implementations have a similar
function, such as
memset_explicit() or
memset_s().
The
explicit_bzero() function addresses a problem that security-conscious
applications may run into when using
bzero(): if the compiler can
deduce that the location to zeroed will never again be touched by a
correct program, then it may remove the
bzero() call altogether.
This is a problem if the intent of the
bzero() call was to erase
sensitive data (e.g., passwords) to prevent the possibility that the data was
leaked by an incorrect or compromised program. Calls to
explicit_bzero() are never optimized away by the compiler.
The
explicit_bzero() function does not solve all problems associated with
erasing sensitive data:
- 1.
- The explicit_bzero() function does not guarantee that
sensitive data is completely erased from memory. (The same is true of
bzero().) For example, there may be copies of the sensitive data in
a register and in "scratch" stack areas. The
explicit_bzero() function is not aware of these copies, and can't
erase them.
- 2.
- In some circumstances, explicit_bzero() can decrease
security. If the compiler determined that the variable containing the
sensitive data could be optimized to be stored in a register (because it
is small enough to fit in a register, and no operation other than the
explicit_bzero() call would need to take the address of the
variable), then the explicit_bzero() call will force the data to be
copied from the register to a location in RAM that is then immediately
erased (while the copy in the register remains unaffected). The problem
here is that data in RAM is more likely to be exposed by a bug than data
in a register, and thus the explicit_bzero() call creates a brief
time window where the sensitive data is more vulnerable than it would
otherwise have been if no attempt had been made to erase the data.
Note that declaring the sensitive variable with the
volatile qualifier
does
not eliminate the above problems. Indeed, it will make them worse,
since, for example, it may force a variable that would otherwise have been
optimized into a register to instead be maintained in (more vulnerable) RAM
for its entire lifetime.
Notwithstanding the above details, for security-conscious applications, using
explicit_bzero() is generally preferable to not using it. The
developers of
explicit_bzero() anticipate that future compilers will
recognize calls to
explicit_bzero() and take steps to ensure that all
copies of the sensitive data are erased, including copies in registers or in
"scratch" stack areas.
bstring(3),
memset(3),
swab(3)